A long time ago, countless projects started using X in a way that wasn't documented (specifically, using X to define multiple modules in one file, instead of one module per file), and it has worked forever just fine. I consider it an unintended (and nice) feature.
X no longer wants to support this use-case, because they discovered that by not supporting it, they can clean up X's code a bit, and it makes X more efficient at certain tasks.
So X turned this into a warning, and is waiting for downstream projects to fix their (new) warnings, before axing this feature. But one popular project, Y, refused to make the change which clears the warning (possibly hoping that X will be more careful to support, not break, idiomatic uses of X).
In the end, project Y voluntarily and without friction added a maintainer who would be willing to keep up with these changes (which were considered by Y to be frivolous). Eventually the system was renamed.
I'd like to add that the fact that you're supposed to use only one DEFSYSTEM form per ASD file wasn't very advertised, at least back when I wrote my first ASDF systems (back when ASDF2 was still in the future) and since then one rarely delved again into full rereading of the manual.
X no longer wants to support this use-case, because they discovered that by not supporting it, they can clean up X's code a bit, and it makes X more efficient at certain tasks.
So X turned this into a warning, and is waiting for downstream projects to fix their (new) warnings, before axing this feature. But one popular project, Y, refused to make the change which clears the warning (possibly hoping that X will be more careful to support, not break, idiomatic uses of X).
In the end, project Y voluntarily and without friction added a maintainer who would be willing to keep up with these changes (which were considered by Y to be frivolous). Eventually the system was renamed.